After a recent issue with my undiagnosed BPD wife heading toward a physical affair: perhaps halted because my son caught her kissing another man, she claims it had only gone that far. Moreover she claims she has never been unfaithful in our 19 years together and until now I have had no reason to believe otherwise.
I like the trust-but-verify approach, and I'd add another couple things.
There aren't very many people who can lie convincingly and consistently, and the more they have to hide and the bigger the things they are hiding, the harder it is to do. More often, if a partner is cheating like this, at some level, the other partner is trying NOT to notice, and not to figure it out, but has an inkling in the back of their head that something is going on.
Does she have a habit of hiding things and being deceptive? And if not with you (that you know of), with other people?
In my case, I'm not a jealous person by my nature, but there were three different guys who twigged my jealousy with regard to me stbexwife. I eventually discovered that one was an emotional affair, and the other two were physical ones. My wife didn't really hide them, but she did try to convince me that all of them were less significant than they actually were to her at the time. You know, she might have even believed what she was trying to convince me of, now that I think of it.
I discovered that my intuition was very accurate, and when I was jealous, there WAS something going on, and I needed to know about it. FYI, stbexw is an incorrigible flirt, and most of the time there was nothing that would ever go beyond flirting, and most of the time it didn't really twig my jealousy very much.
Back to you--Have you felt jealous of your wife around other guys before? And did you feel anything about the guy she kissed? (If you knew he existed... .)